Currently, there are several so-called “last mile” and “last foot” data transmission systems which are designed to deliver high speed and/or high data capacity from the interne backbone to an end user or to provide high speed data communications in a campus-like setting. Several such systems use RF transmissions to replace copper wire or fiber optic cables. Some of these fixed wireless data communications systems are called point-to-point or point-to-multipoint systems and operate in various licensed and unlicensed RF bands. Point-to-point systems, typically have a pair of transceivers communicating only with each other, for example from one building to another. Point to multi-point systems communicate between a hub station, or the like and a number of subscriber stations, or the like. For example, such a point to multi-point system may broadcast from a central tall building to a number of shorter buildings.
Typically either of these systems utilize one frequency band of operation, generally, transmitting and receiving on separate frequencies within the same basic band employing frequency division duplexing (FDD). Such prior art data communication systems operate entirely within one regulated band. These prior art systems employ a subchannel scheme for upstream and downstream in which the frequencies of such subchannels are relatively closely spaced. That adjacency requires extensive use of filters resulting in increased size and cost for data communication systems, particularly for subscriber stations. The band utilized for communication in such prior art systems may be a licensed band or it may be an unlicensed band. Generally, as employed herein, unless otherwise noted, a band is a generally contiguous portion of the electromagnetic spectrum which is regulated by a governmental entity, such as the Federal Communications Commission (FCC) for the United States, generally under a single designation such as those described below.
Another technique which is used to enable communications within a band of operation is time division duplexing (TDD), whereby a base station or hub transmits part of the time on a frequency, and then a subscriber transmits part of the time on the same frequency, thereby sharing the same frequency. Such a system may employ the entire bandwidth available facilitating broadband communications rather than splitting the available bandwidth between upstream and downstream signals as is required in FDD. A disadvantage in a TDD system is that a large amount of coordination is required, resulting in inefficiencies. The bits-per-megahertz efficiency of the data communicated in a TDD system is greatly degraded because time sharing takes a lot of coordination and/or processing power. Further, units in a TDD system are typically not able to operate autonomously, they must be directed from a central unit such as a hub station in order to provide the aforementioned coordination.
In FDD, a base station may simultaneously transmit and receive, and a subscriber station may simultaneously transmit and receive. This provides a benefit in efficiency with this type of system in that the system does not require a lot of processing power or coordination to determine when to transmit and when to receive as with TDD. This inherent efficiency in an FDD system makes good use of spectrum in wireless data communications when larger amounts of spectrum are available. However, the upstream and downstream communications frequencies are typically in the same regulated band. FDD typically needs a reasonable amount, about one percent of bandwidth, of transmission to reception spacing between upstream and downstream frequencies.
In the prior art, the radio of a hub-station or subscriber station may include a duplexer, which employs filters to filter the upstream out of the downstream, and vice versa. In prior art data communications transceivers, a transmitted signal, and even transmit induced noise can enter the transceiver's receiver, because the receiver is intended to receive a very weak signal. So in the prior art very sophisticated filter systems are required to keep the transmit signal or interference caused by the transmit signal out of the receiver.
The Multichannel Multipoint Distribution Service (MMDS) is an FCC regulated communications band that operates in the microwave portion of the radio spectrum, between 2.1 and 2.7 GHz. MMDS. Also known as wireless cable, this band was initially intended for use as a substitute for conventional coaxial cable television. However, the MMDS band has come under wide use for data communication services due to deregulation that allows cable TV companies to provide Internet services. Narrowband channels within MMDS can be used by subscribers to transmit signals to the network. Such narrowband channels were originally intended for use in an educational setting (so-called wireless classrooms). Thusly, the Instructional Television Fixed Service (ITFS) band interleaves with the MMDS band. In ITFS, the FCC allows use of either polarity for transmissions by a license-holder, while in MMDS use of the same spectrum, the FCC requires licensing of the polarity as well as the frequency used for transmission.
Other FCC licensed and regulated bands include the following. Wireless Communications Services (WCS) occupy two fifteen MHz wide bands at 2.3 GHz. WCS is intended for wireless data services. Digital Electronic Message Services (DEMS) at the 24.25 to 24.45 and 25.05 to 25.25 GHz bands offer high data capacity over short distances, particularly useful for providing broadband data services to businesses in dense, urban areas or in a campus environment. Local Multipoint Distribution Services (LMDS) employ the 27.5 to 29.5 GHz and 31.0 to 31.3 GHz bands. Wireless Local Loop and fixed wireless data connection systems such as point-to-point or point-to-consecutive point systems operate in these bands. So called Fixed Wireless Local Loop Services occupy the 38.6 GHz to 40 GHz band.
Problematically, there are only a limited number of licensed bands in any geographic area. Also, in these licensed spectrum bands, limited spectrum problems arise where the licensee may not have as much bandwidth in a particular market as needed to provide broadband services desired by the licensee's customers. Also, problems arise where a licensee has spectrum, but this spectrum is not ideal in terms of a transmit/receive frequency pair. For example, certain amount of spectrum, as a percentage of bandwidth, between the upstream and the downstream (i.e. transmit and receive frequencies) is required to allow state of the art filters to properly operate. Many spectrum licensees do not have sufficient spectrum bandwidth available to provide this separation between employed frequencies.
In 1997 the FCC created a wireless arena called Unlicensed National Information Infrastructure (UNII). System operators are free to operate wireless equipment in three sub-bands (5.15 to 5.25 GHz, 5.25 to 5.35 GHz and 5.725 to 5.825 GHz) without acquiring a licensed frequency spectrum. The FCC specifies the conditions for operating wireless equipment in the UNII frequency band. However, operators are not protected from possible interference from other UNII operators transmitting in the vicinity or even other systems or devices which utilize the same frequencies.
A problem that many wireless data communication system operators face is a need to provide the highest possible data rates to subscriber units. One prior art method of providing greater through-put in licensed bands is to have a very large channel bandwidth providing data to the subscriber. To increase through-put to a particular subscriber unit, the bandwidth must necessarily increase or the modulation scheme that is used has to become more complicated. Problematically, bandwidth is limited in both the licensed and unlicensed bands and using a higher modulation scheme causes an increase in the necessary signal to noise ratio or carrier to interference ratio (C/I). For example, increasing from a quadrature phase shift keying (QPSK) modulation to a 16 quadrature-amplitude modulation (16 QAM), which doubles the throughput, requires a 6 dB increase in C/I. Problematically, such increases in C/I may not be practical, particularly in the unlicensed bands where a significant amount of interference may be present.